1. Field of the Invention
The present invention relates to diesel cycle engines, and more particularly to intake systems for diesel cycle engines. More specifically, the present invention pertains to intake systems having means for improving engine starting and warming up properties, as well as gas drawing systems for introducing into the intake systems gaseous substances such as blow-by gas or exhaust gas.
2. Description of the Prior Art
In diesel cycle engines, efforts have continuously been made to improve starting and warming-up properties. In order to improve the properties, it has been recognized as being effective to increase the temperature of the intake gas as it is drawn into the combustion chamber. In order to accomplish this purpose, proposals have been made to have the intake gas compressed in the final period of the intake stroke to thereby raise the temperature thereof. For example, Japanese utility model application 54-65135 filed on May 16, 1979, and published for public inspection on Nov. 28, 1980, under the disclosure No. 55-165935, teaches to provide the intake passage with an intake throttle valve which is adapted to be closed in the engine starting period and to open the exhaust valve momentarily in the final period of the intake stroke and the beginning period of the compression stroke. With this arrangement, a strong suction pressure is produced in the combustion chamber in the intake stroke due to the influence of the throttle valve so that the exhaust gas is rapidly drawn into the combustion chamber as soon as the exhaust valve is opened in the final period of the intake stroke. The exhaust gas thus drawn into the combustion chamber functions to compress the intake air to thereby increase the temperature of the intake air. The temperature of the exhaust gas itself also has an effect of increasing the temperature of the intake air. A similar system is also disclosed by Japanese utility model disclosure No. 56-154538, which has been published on Nov. 18, 1981.
It has been reported by Hiroshi Kanesaka et al. in the 14th CIMAC Helsinki Conference that this type of intake system has shown significantly improved starting and warming-up properties. It should, however, be noted that the proposed system is disadvantageous in that there is a high possibility that the exhaust gas is drawn back into the combustion chamber not only in the starting and warming-up periods but also during other engine operating conditions. According to the proposals made by the aforementioned utility models, the exhaust cam for actuating the exhaust valve is formed with an auxiliary lobe in addition to a main lobe, so that the exhaust valve is opened by the main lobe to provide the exhaust stroke and by the auxiliary lobe in the final period of the intake stroke and the beginning period of the compression stroke to have the exhaust gas drawn back to the combustion chamber. In this structure, it is understood that the auxiliary lobe on the exhaust cam operates throughout the engine operation so that the exhaust valve is opened in the final period of the intake stroke even when the engine is operated under load. Since the exhaust gas pressure is higher than the intake gas pressure even when the intake throttle valve is wide open, it is very likely that the exhaust as is drawn back into the combustion chamber under heavy load engine operation, causing a decrease in the engine output.
It is, of course, possible to design the exhaust valve operating mechanism so that the auxiliary lobe is effective only in the starting and warming-up periods. However, this solution is undesirable because the valve actuating mechanism becomes very complicated and unreliable.
It should further be noted that the intake throttle valve, which is closed in the starting and warming-up periods, causes further problems in the case where the engine has a gas drawing system for drawing blow-by gas into the intake system in order to prevent the blow-by gas from being discharged into the atmosphere, or for recirculating a controlled amount of exhaust gas into the intake system for the purpose of decreasing nitrogen oxides in the exhaust gas. The blow-by gas or the exhaust gas drawn to the combustion chamber through the gas drawing systems may weaken the suction pressure produced in the combustion chamber so the speed of the exhaust gas flow is decreased to an extent that the compression effect is undesirably weakened. Further, when the blow-by gas is drawn by strong suction pressure, it will be introduced into the combustion chamber without being sufficiently separated from liquid oil. It should further be noted that, in a diesel engine having an exhaust gas recirculation system which is operated during the engine starting period as well for raising the intake gas temperature, the amount of recirculated exhaust gas will be undesirably increased under the influence of the suction pressure, causing a shortage of oxygen in the intake gas.